This invention relates to a process for the continuous foaming of plastics, especially, but not exclusively, for the production of blocks of foamed polyurethane and the like.
In said processes, the mixture of starting materials intended to react and to foam is poured onto a flexible material band advancing on a belt conveyor, defining the base of the blocks to be produced, and bent in a vertical direction to define the sides of the blocks, or accompanied by separate bands defining said sides, which band, in some cases, is further accompanied by another band which defines the upper surface of the blocks, These bands delimit the space in which the foaming of the reacting mixture takes place and thus define the shape in which the mixture solidifies. As soon as a sufficient solidification is attained, said flexible bands are detached from the material which still advances in a continuous manner, and then this latter is cut into blocks of desired dimensions which are transferred onto special shelvings where the reactions being carried out are completed. Usually, the bands used for the purpose specified hereinabove are bands of paper. Although the paper is treated in a special manner to limit its permeability, considerable amounts of the mixture poured are absorbed by the paper and are then removed together with the paper, with the double disadvantage of giving rise to a waste of material and to a pollution of the paper, which therefore cannot even be used as paper pulp in paper mills and consequently involves disposal problems. In the cases in which the conveyor belt is heated to reduce the thickness of the unfoamed skin which forms at the base of the blocks, the skin can achieve such an adhesion to the paper as to give rise to peeling of the skin from the block at th moment of removal of the paper, thus resulting in a further waste of material. Also, under certain circumstances a sweating of substances through the paper takes place, resulting in fouling the installations.
To remedy these disadvantages, attempts have been made to use impermeable bands, for example of polyethylene, instead of the paper bands, but the results do not appear to be satisfactory because of the insufficient stiffness of these bands; on the other hand, the recovery of the material of these bands, which is indispensable in view of their cost, involves considerable technical problems.
Attempts have also been made to superimpose onto a paper band coming from a coil, a relatively thin band of polyethylene coming from a separate coil, but it has turned out that, under conditions of industrial production, it is not possible to ensure that the thin band of polyethylene adheres flatly and without ripples to the underlying band of paper in spite of the stresses which are to be applied for making the bands advance.